Law of superposition and relative dating

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Most fossils are bones or teeth due to their relative hardness. Most of these organisms are very small, like insects, and need to be completely enclosed in a substance like tree sap (like in the movie ), a tar pit, or water that turns to ice.Finding complete fossils like this is even rarer than finding parts of organisms.Unlike people, you can’t really guess the age of a rock from looking at it.Yet, you’ve heard the news: Earth is 4.6 billion years old. That corn cob found in an ancient Native American fire pit is 1,000 years old. Geologic age dating—assigning an age to materials—is an entire discipline of its own.To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This rule is common sense, but it serves as a powerful reference point.Geologists draw on it and other basic principles ( to determine the relative ages of rocks or features such as faults.Layers of volcanic ash are igneous deposits, while layers of rock these deposits surround are usually sedimentary. Igneous intrusions form when magma breaks through a layer of rock from beneath, or lava flows down from above. When igneous intrusion causes newer sedimentary layers to sink into older ones, it's called subsidence.When they break and engulf chunks of sedimentary rocks, it's called stoping. The original rock layers around subsidence areas are called wall rocks and the layers that xenoliths came from are called parent rocks.

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Fossils can give clues to the past and history of life on Earth.Relative age dating also means paying attention to crosscutting relationships.Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type. Sedimentary rocks form from soil and silt carried and deposited by moving water.Over time, the accumulated deposits compress and harden.

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